This application relates to optical and photonic devices for various applications, such as optical, radio frequency (RF), and microwave applications.
Optical devices may be used to manipulate or control light for applications where signals are not in the optical frequencies. As an example, RF and microwave oscillators for generating signals in the RF and microwave frequencies may be constructed as “hybrid” devices by using both electronic and optical components to form opto-electronic oscillators (“OEOs”). See, e.g., U.S. Pat. Nos. 5,723,856, 5,777,778, 5,929,430, and 6,567,436. Such an OEO includes an electrically controllable optical modulator and at least one active opto-electronic feedback loop that comprises an optical part and an electrical part interconnected by a photodetector. The opto-electronic feedback loop receives the modulated optical output from the modulator and converted it into an electrical signal to control the modulator. The feedback loop produces a desired long delay in the optical part of the loop to suppress phase noise and feeds the converted electrical signal in phase to the modulator to generate and sustain both optical modulation and electrical oscillation in RF or microwave frequencies when the total loop gain of the active opto-electronic loop and any other additional feedback loops exceeds the total loss. The generated oscillating signals are tunable in frequency and can have narrow spectral linewidths and low phase noise in comparison with the signals produced by other RF and microwaves oscillators.
Coupled opto-electronic oscillators (“COEOs”) are special OEOs that include both a laser optical loop to produce laser light and an opto-electronic loop that produces an electrical oscillation at a frequency much lower than the optical frequencies, such as a microwave or RF frequency. The laser oscillation in the optical loop is directly coupled to the electrical oscillation in an opto-electronic feedback loop. The laser oscillation and the electrical oscillation are correlated with each other so that both the modes and stability of one oscillation are coupled with those of the other oscillation. U.S. Pat. Nos. 5,929,430, and 6,567,436 describe various implementations of COEOs and are incorporated herein by reference as part of the specification of this application. Like other OEOs, a COEO is tunable in frequency to produce a tunable electrical oscillation in, e.g., RF or microwave frequencies.
The optical loop in a COEO supports different optical modes and the laser oscillation occurs in one or more optical modes. When the COEO is tuned in frequency, the laser oscillation may hop from one optical mode to another of the optical loop. Such mode hopping in the optical loop creates a discontinuity in frequency in tuning and is undesirable in various applications that require continuous frequency tuning.